Management of bird species that lack sexual dimorphism often faces a problem of sex determination. Ordinary methods imply catching birds, that may be difficult in large enclosures, with further taking blood samples or feathers for kariotypes or for cloak inspection, that are at least unpleasant or even traumatic and stressful. In contrast, acoustic method is completely non-invasive and, in some cases, is even more reliable.

In present study we investigated sexual differences in the loud whistles of white-faced whistling duck. Whereas sex of some bird species that lack sexual dimorphism may be recognised in breeding period, both sexes of whistling ducks share common behaviour during the breeding period (sitting on eggs and offsprings’ care). The loud whistles were tape-recorded in Moscow Zoo from 9 adult males and 2 females that were individually marked and were held together in outdoor enclosure. Sex control was made using cloak inspection technique. The loud whistle is a tonal call with well-expressed peaks of frequency modulation that form three maxima and two minima throughout the call. Five temporal and seven frequency measurements were taken from 152 male and 42 female calls.

Discriminate function analysis of sex differences showed 100% correct assignment. The 100% correct assignment was observed even in the case when all frequency or all temporal parameters were omitted from the analysis. Moreover, six out of seven frequency parameters (start frequency and each frequency maxima and minima) showed clear distinction between sexes, without any overlapping or boundary values. The male loud whistles were always lower in frequency than female calls. Mann-Whitney U-test showed significant differences (U=0, p<0.001) for each of six frequency parameters. These differences potentially allow recognition of birds sex by an observer with unarmed ear and provide a reliable tool for non-invasive sex determination in the white-faced whistling duck. The results allowed us to correct ambiguous data received after cloak inspection for one individual. With this method male penis sometimes remains inside the body and sex of this bird was determined as a female under a question. Acoustic data corrected this mistake, suggesting that this bird was undoubtedly male.